Filioque is a theological formula of great dogmatic and historical importance. On the one hand, it expresses the Procession of the Holy Ghost from both Father and Son as one Principle; on the other, it was the occasion of the Greek schism. Both aspects of the expression need further explanation.

I. DOGMATIC MEANING OF FILIOQUI.—-The dogma of the double Procession of the Holy Ghost from Father and Son as one Principle is directly opposed to the error that the Holy Ghost proceeds from the Father, not from the Son. Neither dogma nor error created much difficulty during the course of the first four centuries. Macedonius and his followers, the so-called Pneumatomachi, were condemned by the local Council of Alexandria (362) and by Pope St. Damasus (378) for teaching that the Holy Ghost derives His origin from the Son alone, by creation. If the creed used by the Nestorians, which was composed probably by Theodore of Mopsuestia, and the expressions of Theodoret directed against the ninth anathema by Cyril of Alexandria, deny that the Holy Ghost derives His existence from or through the Son, they probably intend to deny only the creation of the Holy Ghost by or through the Son, inculcating at the same time His procession from both Father and Son. At any rate, if the double Procession of the Holy Ghost was discussed at all in those early times, the controversy was restricted to the East and was of short duration. The first undoubted denial of the double Procession of the Holy Ghost we find in the seventh century among the heretics of Constantinople when St. Martin I (649-655), in his synodal writing against the Monothelites, employed the expression "Filioque". Nothing is known about the further development of this controversy; it does not seem to have assumed any serious proportions, as the question was not connected with the characteristic teaching of the Monothelites. In the Western church the first controversy concerning the double Procession of the Holy Ghost was conducted with the envoys of the Emperor Constantine Copronymus, in the Synod of Gentilly near Paris, held in the time of Pepin (767). The synodal Acts and other sources of information do not seem to exist. At the beginning of the ninth century, John, a Greek monk ie monastery of St. Sab Charged the monks of Mt. Olivet with heresy, because they had inserted the Filioque into the Creed. In the second half of the same century, Photius the successor of the unjustly deposed Ignatius, Patriarch of Constantinople (858), denied the Procession of the Holy Ghost from the Son, and opposed the insertion of the Filioque into the Constantinopolitan Creed. The same position was maintained towards the end of the tenth century by the Patriarchs Sisinnius and Sergius, and about the middle of the eleventh century by the Patriarch Michael Crularius, who renewed and completed the Greek schism. The rejection of the Filioque, or of the dogma of the double Procession of the Holy Ghost from the Father and Son, and the denial of the primacy of the Roman Pontiff constitute even today the principal errors of the Greek Church. While outside the Church doubt as to the double Procession of the Holy Ghost grew into open denial, inside the Church the doctrine of the Filioque was declared to be a dogma of faith in the Fourth Lateran Council (1215), the Second Council of Lyons (1274), and the Council of Florence (1438-1445). Thus the Church proposed in a clear and authoritative form the teaching of Sacred Scripture and tradition on the Procession of the Third Person of the Holy Trinity.

As to Sacred Scripture, the inspired writers call the Holy Ghost the Spirit of the Son (Gal., iv, 6), the Spirit of Christ (Rom., viii, 9), the Spirit of Jesus Christ (Phil., i, 19), just as they call Him the Spirit of the Father (Matt., x, 20) and the Spirit of God (I Cor., ii, 11). Hence they attribute to the Holy Ghost the same relation to the Son as to the Father. Again, according to Sacred Scripture, the Son sends the Holy Ghost (Luke, xxiv, 49; John, xv, 26; xvi, 7; xx, 22; Acts, ii, 33; Tit., iii, 6), just as the Father sends the Son (Rom., viii, 3; etc.), and as the Father sends the Holy Ghost (John, xiv, 26). Now, the "mission" or "sending" of one Divine Person by another does not mean merely that the Person said to be sent assumes a particular character, at the suggestion of Himself in the character of Sender, as the Sabellians maintained; nor does it imply any inferiority in the Person sent, as the Arians taught; but it denotes, according to the teaching of the weightier theologians and Fathers, the Procession of the Person sent from the Person Who sends. Sacred Scripture never presents the Father as being sent by the Son, nor the Son as being sent by the Holy Ghost. The very idea of the term "mission" implies that the person sent goes forth for a certain purpose by the power of the sender, a power exerted on the person sent by way of a physical impulse, or of a command, or of prayer, or finally of production; now, Procession, the analogy of production, is the only manner admissible in God. It follows that the inspired writers present the Holy Ghost as proceeding from the Son, since they present Him as sent by the Son. Finally, St. John (XVI, 13-15) gives the words of Christ: "What things soever he [the Spirit] shall hear, he shall speak; ... he shall receive of mine, and shall shew it to you. All things whatsoever the Father hath, are mine." Here a double consideration is in place. First, the Son has all things that the Father hath, so that He must resemble the Father in being the Principle from Which the Holy Ghost proceeds. Secondly, the Holy Ghost shall receive "of mine" according to the words of the Son; but procession is the only conceivable way of receiving which does not imply dependence or inferiority. In other words, the Holy Ghost proceeds from the Son.

The teaching of Sacred Scripture on the double Procession of the Holy Ghost was faithfully preserved in Christian tradition. Even the Greek schismatics grant that the Latin Fathers maintain the Procession of the Holy Ghost from the Son. The great work on the Trinity by Petavius (Lib. VII, cc. iii sqq.) develops the proof of this contention at length. Here we mention only some of the later documents in which the patristic doctrine has been clearly expressed: the dogmatic letter of St. Leo I to Turribius, Bishop of Astorga, Ep. XV, c. i (447); the so-called Athanasian Creed; several councils held at Toledo in the years 447, 589 (III), 675 (XI), 693 (XVI); the letter of Pope Hormisdas to the Emperor Justinus, Ep. lxxix (521); St. Martin I's synodal utterance against the Monothelites, 649-655; Pope Adrian I's answer to the Caroline Books, 772-795; the Synods of Merida (666), Braga (675), and Hatfield (680); the writing of Pope Leo III (d. 816) to the monks of Jerusalem; the letter of Pope Stephen V (d. 891) to the Moravian King Suentopolcus (Suatopluk), Ep. xiii; the symbol of Pope Leo IX (d. 1054); the Fourth Lateran Council, 1215; the Second Council of Lyons, 1274; and the Council of Florence, 1439. Some of the foregoing conciliar documents may be seen in Hefele, "Conciliengeschichte" (2d ed.), III, nn. 109, 117, 252, 411; cf. P.G., XXVIII, 1567 sqq. Bessarion, speaking in the Council of Florence, inferred the tradition of the Greek Church from the teaching of the Latin; since the Greek and the Latin Fathers before the ninth century were members of the same Church, it is antecedently improbable that the Eastern Fathers should have denied a dogma firmly maintained by the Western. Moreover, there are certain considerations which form a direct proof for the belief of the Greek Fathers in the double Procession of the Holy Ghost. First, the Greek Fathers enumerate the Divine Persons in the same order as the Latin Fathers; they admit that the Son and the Holy Ghost are logically and ontologically connected in the same way as the Son and the Father [St. Basil, Ep. cxxv; Ep. xxxviii (alias xliii) ad Gregor. fratrem; "Adv. Eunom.", I, xx, III, sub init.]. Second, the Greek Fathers establish the same relation between the Son and the Holy Ghost as between the Father and the Son; as the Father is the fountain of the Son, so is the Son the fountain of the Holy Ghost (Athan., Ep. ad Serap., I, xix, sqq.; "De Incarn.", ix; Orat. iii, adv. Arian., 24; Basil, "Adv. Eunom.", v, in P.G., XXIX, 731; ‚Ä¢ cf. Greg. Naz., Orat. xliii, 9). Third, passages are not wanting in the writings of the Greek Fathers in which the Procession of the Holy Ghost from the Son is clearly maintained: Greg. Thaumat., "Expos. fidei sec.", vers. sc. IV, in Rufinus, Hist. Eccl., VII, xxv; Epiphan., Haer., c. lxii, 4; Greg. Nyss., Hom. in in orat. domin. (cf. Mai, "Bibl. nova Patrum", IV, 40 sqq.); Cyril of Alexandria, "Thes.", ass. xxxiv; the second canon of a synod of forty bishops held in 410 at Seleucia in Mesopotamia (cf. Lamy, "Concilium Seleuciae et Ctesiphonte habitum a. 410", Louvain, 1869; Hefele, "Conciliengeschichte", II, 102 sqq.); the Arabic version of the Canons of St. Hippolytus (Haneberg, "Canones Sti. Hyppolyti", Munster, 1870, 40, 76); the Nestorian explanation of the Symbol (cf. Badger, "The Nestorians", London, 1852, II, 79; Cureton, "Ancient Syriac Documents Relative to the Earliest Establishment of Christianity in Edessa", London, 1864, 43; "The Doctrine of Addai, the Apostle", ed. Phillips, London, 1876). The only Scriptural difficulty deserving our attention is based on the words of Christ as recorded in John, xv, 26, that the Spirit proceeds from the Father, without mention being made of the Son. But in the first place, it cannot be shown that this omission amounts to a denial; in the second place, the omission is only apparent, as in the earlier part of the verse the Son promises to "send" the Spirit. The Procession of the Holy Ghost from the Son is not mentioned in the Creed of Constantinople, because this Creed was directed against the Macedonian error against which it sufficed to declare the Procession of the Holy Ghost from the Father. The ambiguous expressions found in some of the early writers of authority are explained by the principles which apply to the language of the early Fathers generally.

II. HISTORICAL IMPORTANCE OF THE FILIOQUE—It has been seen that the Creed of Constantinople at first declared only the Procession of the Holy Ghost from the Father; it was directed against the followers of Macedonius who denied the Procession of the Holy Spirit from the Father. In the East, the omission of Filioque did not lead to any serious misunderstanding. But conditions were different in Spain after the Goths had renounced Arianism and professed the Catholic faith in the Third Synod of Toledo, 589. It cannot be ascertained who first added the Filioque to the Creed; but it appears to be certain that the Creed, with the addition of the Filioque, was first sung in the Spanish Church after the conversion of the Goths. In 796 the Patriarch Paulinus of Aquileia justified and adopted the same addition at the Synod of Friaul, and in 809 the Council of Aachen appears to have approved of it. The decrees of this last council were examined by Pope Leo III, who approved of the doctrine conveyed by the Filioque, but gave the advice to omit the expression in the Creed. The practice of adding the Filioque was retained in spite of the papal advice, and about the middle of the eleventh century it had gained a firm foothold in Rome itself. Scholars do not agree as to the exact time of its introduction into Rome, but most assign it to the reign of Benedict VIII (1014-15). The Catholic doctrine was accepted by the Greek deputies who were present at the Second Council of Lyons, in 1274, and at the Council of Florence, in 1439, when the Creed was sung both in Greek and Latin, with the addition of the word Filioque. On each occasion it was hoped that the Patriarch of Constantinople and his subjects had abandoned the state of heresy and schism in which they had been living since the time of Photius, who about 870 found in the Filioque an excuse for throwing off all dependence on Rome. But however sincere the individual Greek bishops may have been, they failed to carry their people with them, and the breach between East and West continues to this day. It is a matter for surprise that so abstract a subject as the doctrine of the double Procession of the Holy Ghost should have appealed to the imagination of the multitude. But their national feelings had been aroused by the desire of liberation from the rule of the ancient rival of Constantinople; the occasion of law-fully obtaining their desire appeared to present itself in the addition of Filioque to the Creed of Constantinople. Had not Rome overstepped her rights by disobeying the inj unction of the Third Council, of Ephesus (431), and of the Fourth, of Chalcedon (451)? It is true that these councils had forbidden to introduce another faith or another Creed, and had imposed the penalty of deposition on bishops and clerics, and of excommunication on monks and laymen for transgressing this law; but the councils had not forbidden to explain the same faith or to propose the same Creed in a clearer way. Besides, the conciliar decrees affected individual transgressors, as is plain from the sanction added; they did not bind the Church as a body. Finally, the Councils of Lyons and Florence did not require the Greeks to insert the Filioque into the Creed, but only to accept the Catholic doctrine of the double Procession of the Holy Ghost.